ボックスフィルは住宅・軽商業の電気工事で最も見落とされやすい計算です。許容電流や電圧降下は性能に直結しますが、ボックスの過密は仕上げや検査まで見過ごされがちです。
本ガイドでは電線、デバイスヨーク、接地線、内部クランプの数え方を解説します。
Code References
NEC 314.16(A)、314.16(B)、314.16(C) を参照。
なぜ重要か
NEC は各電線・機器に十分な空き容積を要求。曲げ余裕、放熱、保守のためです。
電線サイズと機器に基づく計数方法が提供されています。
12 AWG で 9 許容量なら、ラベルで 20.25 立方インチ以上を確認。 — Hommer Zhao, Technical Director
NEC 計算式
必要容積 = 許容量合計 × 最大電線の立方インチ
難しいのは掛け算でなく正しい数え方。接地、クランプ、ヨークの追加を忘れやすい。
容積許容量
NEC 314.16(B) が各サイズに許容量を割り当て。
| Conductor Size | Volume Allowance | Typical Use |
|---|---|---|
| 18 AWG | 1.50 cu in | Class 1 and fixture wiring |
| 16 AWG | 1.75 cu in | Limited control circuits |
| 14 AWG | 2.00 cu in | 15A lighting and receptacle circuits |
| 12 AWG | 2.25 cu in | 20A branch circuits |
| 10 AWG | 2.50 cu in | 30A equipment circuits |
| 8 AWG | 3.00 cu in | Feeders and heavy loads |
| 6 AWG | 5.00 cu in | Large feeders and service work |
1つの許容量とは
ボックス外から来て終端・通過する各絶縁電線。
- 各絶縁電線は1回。
- 全接地線は最大線で1つ。
- 内部クランプは最大線で1つ。
- 各ヨークは最大線で2つ。
- ボックス内ピグテールは不算入。
Common Misread
二連コンセントは2つの許容量。別ヨークは別カウント。
接地線6本でも1許容量。でもスペースが十分とは限らない。 — Hommer Zhao, Technical Director
計数手順
- ボックス容量を確認。
- 全絶縁電線をリスト。
- 全接地で1つ追加。
- 内部クランプで1つ追加。
- 各ヨークで2つ追加。
- 最大電線の許容量を乗算。
- 必要容積とボックス容積を比較。
計算例
一般的な住宅用ボックスの例。
例1：単極スイッチ、14 AWG
14/2 NM 2本で4本 + 接地1 + ヨーク2 = 7。14 AWG で 14.0 立方インチ必要。
例2：20A GFCI、12 AWG
12/2 2本で4 + 接地1 + クランプ1 + ヨーク2 = 8。最低 18.0。
例3：3路スイッチ
5本 + 接地1 + ヨーク2 = 8。16.0 必要。
例4：2連ボックス
12/2 3本で6 + 1 + 4 = 11。24.75 必要。
例5：ジャンクションのみ
10 AWG 4本 + 接地1 = 5。12.5 必要。
2連で10許容量超えたら大きいボックス指定。追加コストはコールバックより安い。 — Hommer Zhao, Technical Director
ボックスフィル vs コンジットフィル
コンジットフィルは NEC 第9章、ボックスフィルは NEC 314.16。問題が異なる。
よくある間違い
- ヨーク許容量の無視。
- 内部クランプ忘れ。
- ピグテールと通過線の混同。
- 12 AWG に浅型ボックス。
- エクステンションリングの過信。
- 許容電流との混同。
Practical Rule
ぎりぎりで大きな機器を入れるなら、大きいボックスが最善。
エンジニアとDIYへ
設計段階でボックスフィルを確認。
電線サイズと合わせてボックスも確認。
FAQ
全接地線を数えますか？
いいえ。全部で最大線1つ。
コンセントは何個分？
1ヨーク2つ。
ピグテールは？
ボックス内は数えない。
12/2 2本+スイッチの箱は？
7許容量。2.25で15.75。クランプ付18.0。
エクステンションで容量増加？
認定品で表示あれば可。
IEC規格でも？
地域の法規による。原則は同じ。
まとめ
チェックリスト：導線、接地、クランプ、ヨーク、許容量。
導線・管路・電圧降下は当サイトの計算器で。 Contact us
ボックスフィル計算ガイド: Field Verification Table
Before you close out ボックスフィル計算ガイド, it helps to cross-check the same five items that inspectors and experienced installers review in the field: load basis, breaker protection, voltage drop, derating, and grounding or enclosure space. The underlying logic is consistent across the National Electrical Code and the International Electrotechnical Commission: use the actual load, verify the conductor against installation conditions, and only then lock in protection and layout details.
| Design Check | What to Verify | Practical Number | Typical Code Reference | Best Tool or Follow-Up |
|---|---|---|---|---|
| Load Basis | Start from nameplate load, calculated load, or connected VA before picking a conductor. | Continuous loads are usually checked at 125%. | NEC 210.19(A)(1) and 215.2(A)(1) | Use the main wire gauge calculator for the first pass. |
| Breaker Match | Protect the conductor ampacity instead of assuming the breaker sets wire size by itself. | 16A continuous becomes a 20A conductor check. | NEC 240.4 and 240.6(A) | Compare against the breaker sizing guide before trim-out. |
| Voltage Drop | Long runs often require larger wire even when ampacity already passes. | Design target is about 3% branch and 5% feeder plus branch. | NEC informational notes to 210.19 and 215.2 | Run a second check in the voltage drop calculator. |
| Derating | Account for ambient temperature, rooftop heat, and more than three current-carrying conductors. | 90 C insulation may still terminate on a 75 C or 60 C limit. | NEC 310.15 and Table 310.16 | Confirm with the ampacity calculator before ordering wire. |
| Grounding and Fill | Check equipment grounds, conduit fill, and box space as separate calculations. | A 60A feeder often uses a 10 AWG copper EGC under NEC 250.122. | NEC 250.122, 314.16, and Chapter 9 | Cross-check the ground wire and conduit fill guides before inspection. |
“If a circuit will run for 3 hours or more, I treat the 125% continuous-load check as non-negotiable. A 16A design current turning into a 20A conductor decision is exactly the kind of detail that prevents nuisance heat and callbacks.”
“Once branch-circuit voltage drop gets close to 3%, I stop debating and price the next conductor size. Moving from 12 AWG to 10 AWG on a 120V run is usually cheaper than troubleshooting low-voltage performance later.”
“The breaker, phase conductor, and equipment ground are related, but they are not the same calculation. I may upsize a 60A feeder to 4 AWG copper for distance and still keep the grounding conductor at 10 AWG copper because NEC 250.122 keys it to the overcurrent device.”
How to Use This With the Calculator
The calculator gives you a fast starting point, but serious installations still need one more pass for voltage drop, conductor temperature rating, and code-specific exceptions. That last review is where most inspection problems get removed before material is pulled.
ボックスフィル計算ガイド: Practical Number Checks
The easiest way to keep ボックスフィル計算ガイド practical is to sanity-check a few common field numbers before you order wire or close walls. On a 120V branch circuit carrying a 16A continuous load, the 125% rule pushes the conductor check to 20A. That is why 12 AWG copper becomes the real starting point instead of 14 AWG, even before you think about distance. If that same run stretches to 110 feet one way, voltage drop often pushes the design to 10 AWG while the breaker stays at 20A because the load has not changed.
The same logic shows up in larger work. A 7.5 HP, 460V three-phase motor with a full-load current around 11A does not mean you can stop at an 11A wire decision. Motor circuits, feeder calculations, and equipment grounding all apply their own code logic, and the conductor selected from ampacity tables still has to survive ambient temperature, rooftop heat, or bundling. That is why experienced electricians compare the load calculation against conductor ampacity, then against raceway or box space, and only then against the final breaker or fuse size.
Residential work needs the same discipline. A box-fill calculation that lands at 24.75 cubic inches on a 12 AWG two-gang box, or a detached garage feeder that picks up 3.6V of drop on a 120V leg, is already telling you the installation is too close to the edge. Use the long-distance wire guide when length is the problem, and cross-check enclosure constraints with the box fill guide or the conduit fill guide. Those second-pass checks are where most field rework gets avoided.
ボックスフィル計算ガイド: Frequently Asked Questions
How do I know when ボックスフィル計算ガイド needs a larger conductor than a simple chart shows?
If the run is long, the load is continuous for 3 hours or more, or the conductors are bundled in hot ambient conditions, the simple chart is only the starting point. A 20A circuit may still need 10 AWG instead of 12 AWG once the 125% rule or a 3% voltage-drop target is applied.
Does the 125% continuous-load rule matter for ボックスフィル計算ガイド?
Yes, whenever the load is expected to run at maximum current for 3 hours or more. Under NEC 210.19(A)(1) and 215.2(A)(1), a 24A continuous load is treated as 30A for conductor sizing, which is why field calculations often move up one breaker and wire size from the first rough estimate.
What voltage-drop target is practical when planning ボックスフィル計算ガイド?
The common design target is about 3% on a branch circuit and 5% total for feeder plus branch circuit. That is not a mandatory blanket rule in every NEC application, but it is the benchmark many electricians use to decide when a 100-foot to 200-foot run should be upsized.
Can I upsize wire without increasing breaker size for ボックスフィル計算ガイド?
Yes. Upsizing for voltage drop or future durability does not automatically require a larger breaker. A common example is a 20A circuit that moves from 12 AWG to 10 AWG copper on a long run while the breaker remains 20A because the load and overcurrent protection have not changed.
Which code checks should I finish before calling ボックスフィル計算ガイド complete?
At minimum, verify conductor ampacity in NEC Table 310.16, breaker protection in NEC 240.4 and 240.6, voltage drop design assumptions, grounding in NEC 250.122, and enclosure or raceway space in NEC 314.16 or Chapter 9. For international work, align the same review with IEC-style conductor and protection practices.
Next Steps
If you want to validate this topic against real project numbers, start with the wire gauge calculator, then cross-check longer runs in the voltage drop calculator, and verify conductor adjustments with the ampacity calculator. If you want us to add another worked example or application note, contact us here.